Stochastic nature of incipient plasticity in a body-centered cubic medium-entropy alloy

The stochastic nature of the small-scale incipient plasticity (i.e., “pop-in” behavior manifested as a sudden displacement excursion in the load-displacement curves) in a single grain of the body-centered cubic NbTiZr medium-entropy alloy is explored through a series of nanoindentation tests with three spherical tips having different radii (1.8, 3.4, and 8.2 μm). The maximum shear stress underneath the indenter shows distinct distributions depending on the tip radius, wherein both a narrow high-stress regime and a broad low-stress regime are observed. By recourse to deconvolution of the data and detailed statistical analysis, the two regimes were found to be dominated by homogeneous and heterogeneous yielding mechanisms, respectively. Upon hydrogen charging, the tip radius dependence is sustained, but the fraction of heterogeneous yielding is enhanced, which is due to the hydrogen`s effect in assisting defect formation. The suggested scenarios for how the mechanisms are affected by the stressed volume (or tip radius) and hydrogen-induced structural changes were confirmed by direct observations of the deformation behavior underneath the indenter with cross-sectional transmission electron microscopy made on post-pop-in samples.